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Analytical Chemistry

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Capillary Electrophoresis (CE)

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Capillary electrophoresis, or CE, is a technique used in chemical analysis to separate molecules in an electric field according to size and charge.

Capillary Electrophoresis is performed in a sub-millimeter diameter tube, called a capillary, which contains a flowing electrolyte solution. The sample is injected into the capillary, and an electric field is applied. The molecules are then separated based on the difference in their velocity, which is influenced by charge, size, and the solvent's viscosity. CE is ideal for the separation of charged molecules and has a greater resolution than high-performance liquid chromatography, making it more efficient and sensitive.

This video will introduce the basics of capillary electrophoresis and demonstrate its use by determining the composition of a soft drink.

In CE, an electric field is applied to a capillary filled with an electrolyte. The electric field induces a positive charge at the capillary inlet, and a negative charge at the outlet.

The electrolyte flows within the capillary, induced by the electric field. This flow, called electro-osmotic flow, is caused by the movement of a discrete layer of positively-charged salt ions along the negatively-charged capillary walls.

As the electric current runs through the capillary, the cations along the wall move toward the negative end. This ion flow pulls the solution in the center through the tube.

The sample molecules are then separated based on their velocity within the capillary. This velocity, called electrophoretic mobility, depends on the molecules' charge and size, and how much it is attracted or repelled by an electric field.

Positively-charged molecules flow faster through the capillary, as they are more attracted to the potential at the outlet. Negatively-charged molecules flow much slower, as they are more attracted to the potential at the inlet. Neutral molecules are carried along with the bulk flow. Thus, the order of molecules exiting the capillary is positively-charged, neutral, and then negatively-charged. Additionally, the electrolyte flow pulls smaller molecules faster than larger molecules due to frictional forces.

Molecules are recorded by a detector, such as UV-Vis, as they exit the column, and are visualized in a plot of detector signal intensity versus time, called an electropherogram.

Electropherograms can yield a range of information; such as how many different compounds are present in a sample and the amount of each substance.

Now that you've seen a brief synopsis of capillary electrophoresis, let's take a look at how it is performed in the laboratory.

First, turn on the capillary electrophoresis instrument and computer. Then switch on the UV detector to allow it to warm up.

Set the parameters for running the experiment. First, set the temperature for the cartridge and sample storage to 35 °C and the wavelength for UV detection to 214 nm.

Next, set the two rinse steps. The first rinse is with sodium hydroxide to ensure that the silanol groups on the capillary wall are protonated. The second rinse uses running buffer to equilibrate the capillary. Then, set the sample to inject at 0.5 psi for 5 s.

Set the electrophoresis step, by selecting the separation voltage. In this case, use 20 kV for 5 min using normal polarity, meaning that the electric field is positive at the inlet and negative at the outlet.

First, prepare 50 mL stock solutions of the soda components aspartame, caffeine, and benzoic acid at 500 parts-per-million in water.

From the stock solutions, make a standard solution of 150 ppm aspartame, 150 ppm caffeine, and 100 ppm benzoic acid.

Then, make 4 standard solutions of caffeine at 50, 100, 150, and 200 ppm. These samples will be used to make a calibration curve. Fore more information, see this collection's video on calibration curves.

Finally, prepare the soda samples by degassing them with nitrogen. The soda samples will be analyzed with no dilution.

Place the vials containing either standard or soda samples into the vial holder. Place the vials containing the run buffer and the sodium hydroxide rinse solution into the sample holder as well. Make sure to record the location of each.

In the CE instrument software, indicate which slots contain the two rinse solutions and the first sample vial. Now, run the first sample.

Then, run the combination standard, the 4 concentrations of caffeine, and a regular and diet soda sample by changing the input vial.

When all of the solutions have been separated, analyze the data.

First, use the standards to identify the peaks in the soda samples. A comparison of the three peaks observed in the diet soda sample to the standards shows that caffeine, aspartame, and benzoic acid are present in the diet soda. In the regular soda sample, only the caffeine peak is present, but not the aspartame and benzoic acid peaks.

Then, calculate the peak area for each caffeine standard solution, and make a calibration curve. The calibration curve for caffeine can be used to calculate the concentration of caffeine in each sample.

Capillary electrophoresis is used for many specialty separations in academic and industrial settings.

CE is often used as one component of pharmaceutical industry quality control testing. Drugs, either as small molecules or biologics, can be run through capillary electrophoresis to see if any side products are present. It can also be used to determine whether proteins are properly folded, as folding can affect protein charge.

CE can also be used to separate DNA. Using a microwell plate and multiple arrays of capillaries, researchers can increase the throughput of a single experiment, as shown here. DNA fragments were separated based on size, with a resolution down to 1 base pair. This makes sequencing fragments of DNA possible, along with determining other parameters like copy number variants, which is used to diagnose potential genetic diseases.

A protein can be modified by various functional groups that are chemically attached to different locations. Different copies of the same protein can vary with different modifications, which will change the charge and size of each protein. Running the purified proteins through a CE that is attached to a mass spectrometer can separate proteins based on which modifications are present, and also identify the type and location of the modification.

You've just watched JoVE's introduction to capillary electrophoresis. You should now understand how CE separates molecules based on charge and mass, and how to run a sample on the CE in the lab.

Thanks for watching!

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